FIG. 1 is a schematic circuit diagram showing a typical flyback converter system 1 for controlling a primary side circuit thereof in the prior art. The flyback converter system 1 includes a DC input power source 11, a flyback converter (12 and 14) and an LED diode module 13. The DC input power source 11 provides a DC input voltage Vin. The flyback converter (12 and 14) includes a main circuit 12 and a controller 14. The LED diode module 13 includes a plurality of LEDs. The main circuit 12 of the flyback converter (12 and 14) includes a transformer 121, a main switch S, an output diode D0, an output C0 and a current detecting resistor Rcs. The transformer 121 includes a primary winding 1211, a secondary winding 1212, a magnetizing inductance Lm and an auxiliary winding 1213. The controller 14 includes a current adjuster 141, a zero current detector (ZCD) circuit 142, a pulse width modulation (PWM) generator 143, a voltage adjuster 144, a gate driving circuit 145, a current sensor (CS) 146, a voltage sensor (VS) 147, a controller power source vcc and a ground GND. The primary winding 1211 of the transformer 121 has a primary winding turns number np and a primary winding voltage Vpri. The secondary winding 1212 of the transformer 121 has a secondary winding turns number ns and a secondary winding voltage Vsec. The auxiliary winding 1213 has a winding turns number naux and an auxiliary winding voltage Vaux crossing over. The main switch S has a gate voltage Vgs. The current detecting resistor Rcs has a voltage Vcs. The output capacitor C0 has a voltage Vo, i.e., the flyback converter system 1 has an output voltage Vo. In addition, the magnetizing inductance Lm has a current iLm flowing through; the main switch S has a current Is flowing through; the output diode D0 has a current iD flowing through; and the flyback converter system 1 has an output current Io.
The main circuit 12 feedbacks an output information of the flyback converter (12 and 14), by using the auxiliary winding 1213 and the controller 14, to a primary side circuit, which includes the primary winding 1211, the magnetizing inductance Lm, the main switch S and the current detecting resistor Rcs. The controller 14 modulates a duty cycle of the main switch S to achieve a purpose of controlling method for the output current Io and the output voltage Vo after processing the relevant output information.
FIG. 2 is a timing diagram showing typical waveforms formed in the flyback converter system 1 for controlling the primary side circuit thereof shown in FIG. 1 in the prior art. The output voltage Vo is controlled by using the auxiliary voltage Vaux to control the primary side circuit. The relevant formula is as the following equation (1), where Rd is an equivalent series-connection resistance, VF is a conducting voltage drop of the output diode. Based on FIG. 1 and equation (1), it can be seen that the auxiliary voltage Vaux detected by the auxiliary winding 1213 is affected by a parasitic characteristic of the output diode D0. Therefore, an operating mode for the flyback converter (12 and 14) is always restricted in a DCM (discontinuous current mode) when the primary side circuit is controlled. In FIG. 2, the auxiliary voltage Vaux is detected at the time t=ts in the DCM, wherein the item iD(t)Rd will be zero, the conducting voltage drop VF is about a constant, and the auxiliary voltage Vaux is simplified for a compensation of the output voltage Vo.
                                          v            aux                    ⁡                      (            t            )                          =                                            n              aux                                      n              s                                ⁢                      (                                          V                O                            +                                                                    i                    D                                    ⁡                                      (                    t                    )                                                  ⁢                                  R                  d                                            +                              V                F                                      )                                              (        1        )            
Regarding controlling the output current Io, it can be derived from the following equation (2), by detecting a peak current Ipk at a time the main switch is turned off and by measuring a conducting duration Tdis of the output diode D0. In the equation (2), Ts is a switching cycle of the main switch, and iD is a current flowing through the output diode Do. However, the formula in the equation (2) still needs to be used on the condition that the operating mode of the flyback converter (12 and 14) is restricted to the DCM. Because the current iD flowing through the output diode Do in the Equation (2) is not zero, resulting in another non-zero initial current value Iini existing in the item (IPK/2) of the equation (2) when the main switch S is turned on.
                              I          o                =                                            1                              T                s                                      ⁢                                          ∫                0                                  T                  s                                            ⁢                                                i                  d                                ⁢                dt                                              =                                                    T                dis                                            T                s                                      ⁢                          (                                                I                  PK                                2                            )                        ⁢                                          n                p                                            n                s                                                                        (        2        )            
The technique applied to the primary side circuit can avoid adding control circuit components in the secondary side circuit, reduce component costs effectively and achieve relatively low standby power consumption. However, it is worth thinking deeply about how to improve the technique of controlling the primary side circuit and its relevant controlling method in order to achieve the purpose of controlling the output voltage and the output current whenever the flyback converter system 1 is operated in the DCM or a CCM (continuous current mode).
Therefore, the inventor is in view of drawbacks of the prior art, thinks an idea to improve the prior art, and invents the present application “flyback converter and controlling method thereof”.